Existing standards for the broadcasting of colour television signals (NTSC, SECAM and PAL) take into account the characteristics of the television receiver, particularly with respect to compatibility with monochrome television receivers and availability of established transmission channels.
In order to avoid the operation of transcoding, which involves additional expense and degradation of quality, it has been the usual practice in television studio complexes to transmit or distribute the colour television signals from place to place within the studio complex in the same standard form (NTSC, PAL or SECAM) as that in which they are eventually broadcast in the country concerned. Because the above mentioned colour television standards are basically suitable for the distribution or transmission of colour television signals within the studio, this method has in the past not encountered any serious difficulties. Any particular disadvantages determined by the characteristics of the above mentioned standards, for example problems involved in the mixing of SECAM signals, could be overcome.
With the increasing introduction of digitally operating studio equipment--synchronizers, digital recording devices, digital mixing devices--it has become necessary to an increasing extent to perform the conversion of analog colour television signals into digital signals and to reconvert them. This conversion and reconversion can be necessary at several different stages in the path taken by the signals from the signal source (for example a colour television camera or film scanner) to the broadcast transmitter.
Basically there are two known techniques for performing this conversion, firstly the component encoding, i.e. closed coding technique wherein a colour television signal available according to one of the above standards is sampled as such and digitized, and secondly the component encoding, i.e. open or separate coding technique wherein a colour television signal available according to one of the above standards is divided into its individual luminance and chrominance components and the components are then individually sampled and digitized. Both methods possess disadvantages: in the case of the closed coding technique a high data rate results from the necessary relationship between the sampling frequency and the colour carrier frequency, and this high data rate presents difficulties in recording; on the other hand the separate coding technique requires a division of the analog coded signal, resulting in an inherent loss of quality. Both methods produce a digital data stream which is distributed in the form of either a parallel bit stream or a serial bit stream in the studio. Among the disadvantages of the parallel bit stream is that which occurs, for example, when it is necessary to make a selection among a plurality of digital signal sources, because there may be the necessity of simultaneously switching up to 8 or more lines.